CHICAGO — If you’ve taken the high-speed ferry across Lake Michigan between Milwaukee and Muskegon, Mich., in the past few years, you’ve been part of an experiment.

As the ferry makes its 2 1/2-hour trip across the lake, a device in the engine room measures carbon dioxide dissolved in the surface of the lake, and another sensor on top of the ferry tracks it in the air.

Harvey Bootsma, a researcher at the University of Wisconsin-Milwaukee, has been using the Lake Express ferry as a mobile laboratory since 2006 to tackle an unresolved question: Do the Great Lakes soak up carbon dioxide from the atmosphere, the way forests do? Or do they behave “like we do,” as Bootsma puts it, by exhaling it?

Finding the answer would not only shed light on the lakes’ ecosystems but would also contribute to our understanding of climate change. Scientists say carbon dioxide released through fossil fuel burning contributes to global warming. Carbon sinks — vast natural structures like forests and oceans — can slow the buildup by absorbing carbon dioxide from the air.

The Great Lakes are complex enough that Galen McKinley, assistant professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison, calls them “small oceans as opposed to big lakes.” Yet they are not yet part of current models estimating the impact of natural carbon sources and sinks in the United States.

“They’re significant enough on a regional basis that they should be included in the budget to see what can safely be emitted by humans,” said Noel Urban, professor of civil and environmental engineering at Michigan Technological University.

The challenge is that the lakes are vast — they account for 20 percent of the world’s available freshwater, and 90 percent of that in the U.S. The biological processes that control their carbon dioxide dynamics change from the shore to the open lake, as well as through the seasons. Invasive species and climate change, meanwhile, are changing them from year to year.

To understand the lakes’ role as carbon sources or sinks, researchers must study them throughout the year, in as many places as possible. That is expensive to do by traditional shipboard sampling, and iced-in ports also have kept scientists from measuring the lakes in winter.

“Here’s something right in our backyard and yet we don’t know what’s going on out there for half the year,” Urban said.

Bootsma’s ferry-based system, which plies the lake from May through October, has helped him solve part of the puzzle. In summer, the ferry crosses the lake six times a day. In addition to carbon dioxide, it measures temperature, dissolved oxygen and chlorophyll fluorescence, which indicates the concentration of algae growing in the water.

“It has really improved our insights into the carbon dioxide dynamics of Lake Michigan,” Bootsma said. “There is no way we could collect this type of data using conventional research ships.”

This is the first time a volunteer observation ship has measured carbon dioxide on the Great Lakes, although oceanographers at the National Oceanic and Atmospheric Administration have outfitted container ships with similar equipment to study the oceans.

Bootsma has also installed an automated buoy about a half-mile off Atwater Beach in the village of Shorewood, Wis., near Milwaukee, to study carbon dioxide closer to shore. The buoy has a weather station on top, and every hour it makes the same measurements as the ferry system, as well as for conductivity, pH level and the water’s cloudiness, or turbidity.

So far, Bootsma has found that from late spring through October, Lake Michigan is a sink for carbon, as algae use the abundant light and nutrients to take up carbon the way plants do.

In the fall, this picture reverses itself. Wind and falling temperatures bring the lake’s deep waters to the surface, and with them comes carbon dioxide that has built up deep in the lake, where microbes decompose plant and animal remains. Once the surface water has higher carbon dioxide levels than the air, the gas diffuses out, and the lake becomes a carbon source.

But what happens in winter was a mystery until recently. The ferry stops running in November, and around the same time Bootsma’s group has to remove the buoy from the lake to prevent ice damage.

Last winter, Milwaukee Water Works allowed Bootsma to put his carbon dioxide monitoring device inside its water-treatment plant, which pulls in lake water from about three-quarters of a mile offshore. Early information suggests the lake is a source of carbon dioxide in winter and early spring.

Bootsma’s group is now bringing together the data from the past three years to make a final call on the lake’s effect on the atmosphere over a yearly cycle, and plans to submit it for publication early next year.

Taking on the same question in Lake Superior, McKinley leads a team that includes Urban and other researchers as part of the North American Carbon Program, which aims to determine the continent’s carbon sources and sinks.

Urban’s field studies of Superior suggested that it was a strong source of carbon to the atmosphere, enough to cancel out up to half the value, on a per-square-mile basis, of the sink created by the forests in the watershed.

But most of his measurements were made near shore, which tends to have higher carbon dioxide levels than the open lake because plant matter swept in from the watershed decomposes there.

McKinley thinks the lake is closer to neutral with respect to the atmosphere, based on a 10-year record of carbon dioxide data on the open lake compiled by the Environmental Protection Agency, and her group’s computer model.

Her estimate of the lake’s annual carbon dioxide emissions is equivalent to 1 percent of the carbon dioxide produced by fossil fuel burning each year in Michigan, Wisconsin and Minnesota combined.

But the EPA’s data provide just “two snapshots” a year, in April and August, McKinley said. Without winter data, the lake’s carbon dioxide emissions may be underestimated. And her model doesn’t yet include rivers, a source of carbon to the lake.

The question is far from settled. But the researchers agree that the biology of the lakes — the algae that absorb carbon and the microbes that release it — strongly controls how they process carbon. They believe this effect is more important even than temperature, which affects how carbon dioxide dissolves in water.

As you comment, please be respectful of other commenters and other viewpoints. Our goal with article comments is to provide a space for civil, informative and constructive conversations. We reserve the right to remove any comment we deem to be defamatory, rude, insulting to others, hateful, off-topic or reckless to the community. See our full terms of use here.

More in News

A wintry mix is threatening to make the next few days interesting for Twin Cities drivers. The National Weather Service is predicting multiple rounds of light snow and freezing rain starting early Sunday morning and continuing through midday Tuesday. Clouds will settle in Sunday, bringing a 30 percent chance of snow before noon and a high near 39. By midnight,...

Hundreds of participants took the polar plunge Saturday at Prior Lake to raise money for the Special Olympics. The annual event, sponsored by Special Olympics Minnesota and local law enforcement, raised $158,179 towards programming and events for more than 8,200 special needs athletes statewide. The total was down from last year’s event that raised $192,258. The 568 participants that registered online raised...

Last month, a New Brighton commissioner wrote a letter to the editor of the New Brighton Bulletin questioning the motives of the former city council to change elections from odd years to even years. On Tuesday, the commissioner lost his volunteer position because of that letter. The council voted 3-2 to remove Ben Jones from the planning commission. Council members...

HALLOCK, Minn. — It was an improbable romance between strangers in a faraway place. Nathan Younggren was a 25-year-old farmer who raised wheat, soybeans and beef cattle with his family near Hallock. Victoria “Tori” Allen was a 28-year-old former ski bum and advertising rep who lived in Colorado. He played rock music in a cover band, and she was a...

State health officials say three more children have died of complications of the flu, bringing the number of Minnesota children who have died of influenza this season to four. The number of Minnesotans hospitalized with flu symptoms has topped 4,200. That’s the highest number since the Minnesota Department of Health began tracking flu hospitalizations in 2008. Preliminary figures released Thursday...